Staging: hv: Fix vmbus event handler bug

The flag ENABLE_POLLING is always enabled in original Makefile, but
accidently removed during porting to mainline kernel. The patch fixes
this bug which can cause stalled network communication.  Credit needs to
go to Eric Sesterhenn <eric.sesterhenn@lsexperts.de> For pointing out a
typo in the original code as well.

Signed-off-by: Hank Janssen <hjanssen@microsoft.com>
Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>